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Maybe the most searing image of the flood tragedy that struck Vermont a week ago was the video of the 140-year-old Lower Bartonsville Covered Bridge in southeastern Vermont washing away. The physical power of the torrent was stunning. The bridge sank, defeated but dignified, into the raging brown water like a captain going down with the ship.

The emotional power was just as striking. In the video, you hear Susan Hammond, the woman shooting the images, crying and swearing as the bridge collapsed. You can’t blame her. Vermont’s worst natural disaster since 1927 felt like a rock-hard sucker punch to the gut.

Several days later, Hammond said the whole village of Lower Bartonsville, a section of Rockingham, remained stunned by the loss of the bridge.

“It was devastating,” she said.

That Vermont’s usually scenic, tame waterways could rise up and kill people, shove buildings off their foundations, make miles of state highways disappear in a day and upend everyone’s sense of security is testimony to the extreme intensity of the floodwaters.

The volume of water racing down Vermont rivers and brooks boggles the mind. The peak flow along the White River in West Hartford was 100,000 cubic feet per second, said Andy Nash, head meteorologist at the National Weather Service office in South Burlington. That’s 500 times the normal flow of the White River — and equivalent to the normal volume in the Mississippi River at St. Louis.

“The fact it is approaching or maybe exceeding the ’27 flood in certain areas of the state is what startles me more than anything else,” said Barry Cahoon, a river management engineer with the Vermont Department of Environmental Conservation.

Tranquility broken

In a normal August, the brooks, creeks and small rivers that caused such devastation this week are as placid as can be. Usually, at this time of year, people wade across the brooks and barely get their ankles wet. Toddlers sit in the middle of the streams and serenely build little rock sculptures while their smiling parents bask in the sunshine and watch.

Last Sunday, anyone who got near any of these brooks did so at their own peril. In Waitsfield, I could hear boulders, some probably the size of Volkswagens, rolling and bumping in the Mad River stream bed under the force of the roiling floodwaters. On the Huntington River in Huntington, I saw what looked like part of a deck followed by a blue barrel zoom past in the torrent as if they were participating in a high-speed chase.

Rainfall Sunday generally ranged from 4 to as much as 9 inches in Vermont. Everybody agrees that’s a lot, but at first glance, 6 inches of precipitation doesn’t seem that threatening. It’s enough to fill a drinking glass of water.

But William Bowden, a professor of watershed science and planning at the University of Vermont, puts it this way:

A half foot of water falling on a square mile of land amounts to 14 million cubic feet of water. A swimming pool 100 feet long, 50 feet wide and 6 feet deep holds 30,000 cubic feet of water — so 6 inches of rain falling on a square mile of real estate would fill 467 of these swimming pools.

The Winooski River watershed covers 1,044 square miles, so if an average of six inches of rain fell over the basin, it amounts to about 14 million cubic feet of water, or 108 trillion gallons, Bowden said.

“That’s a half million swimming pools that fell out of the sky in 12 hours,” he said.

Debris, big and small

Hammond watched in horror as that volume of water roared into the Williams River. As the water rose toward the Lower Bartonsville Covered Bridge, debris raced by.

“You could see trees floating in the river with their limbs scraping against the bridge, and logs flipping head over tail in the water,” she said later in an interview with the Burlington Free Press.

Vermont’s rivers snatched huge things and moved them willy-nilly. Crews found a roof on Vermont 100 in Wilmington — and nobody knows where it came from. The Neshobe River in Brandon pushed the Brandon House of Pizza restaurant 20 feet down the street.

The raging rivers also picked up tiny things that also are important. Those include uncountable grains of sediment, thousands of pounds of which ended up in Lake Champlain and other Vermont lakes, Bowden said.

“This much water is well beyond what’s necessary to move tons and tons of sediment,” Bowden said. Sediment carries large quantities of phosphorus, which can hinder lake quality and promote algae blooms. The flood does not auger well for a Lake Champlain that already suffered waves of heavy sedimentation in destructive spring floods, he said.

I was chilled by a YouTube video that popped up the other day. It showed a torrent of water, rocks and mud cascading down a steep hillside and partly burying Vermont 4A, less than a quarter mile from my elderly parents’ house in West Rutland. The water never touched their house, and they’re fine, but that close miss was worrying.

The video is an example of how Vermont’s topography, combined with the widespread nature of the rainfall, enhanced the floodwaters’ power, said Greg Hanson, a hydrologist with the National Weather Service in South Burlington.

Topography matters

Vermont’s steep terrain encourages water from heavy rain to rush downhill rather than soak into the ground. That’s partly why flooding was worse in mountainous regions of the state than it was in the relatively flat Champlain Valley, Hanson said.

Flash floods in Vermont typically come from heavy, localized thunderstorms. These storms can cause serious local damage, but the heaviest rain falls on only part of a drainage basin. That means the wave of water coming downhill becomes less intense, because the streams feeding the wave as it goes by don’t have much excessive water in them. By the time the wave hits populated communities on valley floors, some of their punch is gone, Hanson said.

During the tropical storm, though, the entire state was drenched by torrential rain.

“With Irene, every basin in the state got 6 inches of rain, 8 inches of rain,” he said. “When that happens, the flood wave actually builds.”

As waves of floodwater made their way down mountain slopes, they were joined by torrents coming from other overwhelmed tributaries. By the time the water reached low-lying towns such as Wilmington, Brattleboro and Rochester, it had built to maximum force, he said.

Driving around during the downpours Sunday, I avoided driving onto flooded roads, even if the water seemed shallow. Smart move, Hanson said; seemingly heavy cars are surprisingly buoyant in floods.

“Once the water gets up above the doorsills, the amount of weight is less on the wheels,” he said. “That’s less friction on the tires to keep the car from floating away. It doesn’t take a foot of water, and not a fast current.”

That’s why you saw so many images of cars tossed about in the flood like Lego blocks thrown around a room during a child’s temper tantrum.

Devastating thoughts

In addition to the floodwaters, powerful emotions swept through Lower Bartonsville, as they did through the rest of the state.

“The whole neighborhood was down there throughout the day,” Hammond said, adding that everybody prayed the bridge would somehow, against all odds, hold against the rising water.

“I could not believe it. The bridge was creaking as it was shifting,” Hammond recalled. Then came the brutal CRACK, and the Williams River gobbled up the Lower Bartonsville Covered Bridge. It was all over in six seconds, she said.

“Right now, we’re all talking about a lot of memories, and how we used to race across the bridge, because the older boys used to hide in the bridge rafters to scare us,” Hammond said. “I remember swimming under the bridge, walking across it and looking down and seeing the river.

“It was part of what makes Lower Bartonsville a community,” she continued. “It’s not just a road with 20 houses on it. You enter through that bridge, and it takes you back to the history of the village.”

Rethinking rebuilding

Nobody knows how Lower Bartonsville or the rest of Vermont will look like after we rebuild. Cahoon, the state river management engineer, said given the power of Vermont’s waterways and the power of climate change, we shouldn’t rebuild Vermont the way it was before the flood.

“We can’t approach this with the idea we’re just going to put it back the way it was before,” he said. “It’s not going to work. This is not the last time we’re going to see this. The climate is changing. We’re going to see this more.”

Although a single storm such as Irene is not necessarily proof of global warming, in general, warmer air in a changed climate can hold more moisture. If there’s more moisture, you can get heavier rainstorms and more frequent floods, according to climatologists. There might be evidence this is happening in Vermont.

The state always has had destructive floods. But in recent years, they’ve been more frequent. Serious damage from flash floods has occurred somewhere in Vermont in all but one of the past 15 years. Serious flooding slammed parts of Vermont in April, again in May, and now with Irene in August.

Trying to control the rivers as Vermonters have done since the state was settled will prove fruitless, Cahoon said.

“We drained, straightened and channelized and did everything we could to quote, unquote, improve productivity of the land,” he said.

Road-construction workers shoved rivers up against the sides of valleys. Flood plains were filled in for houses, roads, commerce. All this made rivers deeper and straighter, and left them without space to spread out during floods.

“The power of water is a function of depth and velocity. The deeper you make it, the more power it has,” he said. If you confine a stream you speed it up, and you increase its depth, and that’s what gives it the power to erode its banks,” he said.

Which is why many of Vermont’s rivers ate away at many of the state’s highways last Sunday.

“The stream is just saying, ‘I need space here. You took it away from me. I need to be able to dissipate my energy. You can’t take it away from me because I have more power than you do,’” Cahoon said.

Somehow, he said, Vermont has to try to rebuild in a way that gives the rivers their space.

New Bartonsville Bridge?

In Lower Bartonsville, there’s already talk of replacing the bridge.

The town of Rockingham had a $1 million insurance policy on the bridge’s structure, but not on the more expensive abutments that hold it up, Hammond said.

People who watched Hammond’s video have offered money. Local residents are offering donations and want to organize fundraisers, she said. The town put a link on its home page, www.rockbf.org, soliciting donations. The efforts are helping to cheer Hammond up.

“We have such a sense of community and history,” she said, “and that bridge is such a part of it.”

Which proves, after all, there is indeed something more powerful than Vermont’s worst flood disaster in almost a century.